In optical fiber telecommunications, flexible tubes are often used for identifying, protecting, and guiding fibers over portions of their paths where they are not integrated in a cable.
For example, mention may be made of the connections between grooved-core cables and connection systems in splice boxes a in distribution boxes, outputs of emitters and receivers, etc.
In general, each end of the flexible tube is connected either to the end of a cable or else to a support integrated with a component.
In the particular case of the splaying-out device (DE) of French patent application No. 82/7483, the flexible tubes are fixed to a head for extracting the fibers from the grooved cores; the head is itself fixed to the core and the other ends of the tubes are fixed to a connection box.
The flexible tubes are generally made of extruded thermoplastic materials having a linear expansion coefficient .alpha.L lying in the range 500 to 1,500.times.10.sup.-7 K.sup.-1 (per degree), which is to be compared with the equivalent coefficient of glass which is about 5 to 10.times.10.sup.-7 K.sup.-1. Differential expansion therefore arises which may apply mechanical stresses to the fibers and to the connections.
In normal applications, the flexible tubes are disposed in coils so as to store reserve lengths of fibers. Since the inside diameter of the tubes is not less than two to three times the outside diameter of the fibers, there is theoretically enough free space to take up the differential expansion, provided, that is, that the fiber can slide freely along the tube, and provided it is suitably positioned on assembly.
Where these conditions cannot be satisfied, or where temperature differences are too great, mechanical stresses (bending, traction) are applied to the fiber and to the connections, with the following consequences:
reversible or irreversible signal attenuation; and PA1 danger of fiber breakage, in the long term. PA1 a base material Mb which, after stretching to a ratio Si/Sf=r, has a considerably reduced coefficient of expansion; PA1 a coating material Mc which, after being stretched to the same ratio r, has physical characteristics which are little changed, and an in particular has no tendency to form fibrils; PA1 said two materials adhering well to each other. PA1 a. a two-layer tubular preform is extruded, preferably with both layers being co-extruded in a single pass; PA1 b. the preform is stretched to ratio r at a temperature less than the melting temperature of the base material Mb; and PA1 c. thermal stabilization treatment at a temperature which is at least 10.degree. C. greater than the maximum in-use temperature.
It is known that a stretching treatment at a temperature less than the melting or softening temperature of thermoplastic materials may give rise to a high degree of anisotropy in their physical properties, and in particular in their expansion coefficients. By this means, it is possible to obtain flexible tubes having a longitudinal linear expansion coefficient .alpha.L which is considerably less than that of the initial material.
By way of example, for a flexible tube made of polypropylene, it is possible to obtain values of .alpha.L which are less than 100.times.10.sup.-7 K.sup.-1.
However, it has been observed that when a polypropylene tube is subjected to stretching treatment suitable for obtaining an acceptable coefficient of linear expansion (e.g. less than 400.times.10.sup.-7 K.sup.-1), it becomes highly sensitive to the apperance of surface fibrils. Such fibrils are formed on the surface of the tube, in particular when it is rubbed. They constitute a considerable impediment in the operations of manufacturing the tubes or the splaying-out devices, and also to the use thereof.
The aim of the present invention is to provide a flexible tube of thermoplastic material having a mean linear expansion coefficient which is less than 300.times.10.sup.-7 K.sup.-1 in the temperature range -30.degree. C. to +60.degree. C., and which does not give rise to the formation of surface fibrils during the operations of manufacture and assembly.
It has been observed that there is a critical stretching ratio rc which gives rise to the appearance of fibrils: this critical stretching ratio varies as a function of the nature of the material, and for a given polymer material it varies as a function of its molecular weight.